feat(messaging-ui): Inbox domain — unified work items + mail (contract + UI + mock)

Second domain in the SDK, mirroring messaging's adapter/provider/hooks/components:
- InboxAdapter (listInbox unified feed + transition + mailHistory/mailReply +
  optional directory/composeInternal/composeExternal), InboxProvider/useInboxAdapter
- hooks: useInbox (filter + transition), useMailThread (history + reply), useCompose
- <Inbox> (filter tabs, unified list, detail pane), <MailReader> (HTML in a
  sandboxed iframe + reply), compose modal (in-app / email); themeable styles
- MockInboxAdapter (seeded mentions/needs-reply/alert + folded mail threads +
  directory) shipped from ./adapters/mock-inbox
- 6 inbox tests (mock unify/transition/reply + render open/reply/compose); 64 green

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-messaging-ui/src/index.ts

@@ -17,6 +17,13 @@ export { ChannelBrowser } from './components/channel-browser';
 export { Thread } from './components/thread';
 export { ThreadPane } from './components/thread-pane';
 
+// ── Inbox domain (work items + mail) ──
+export { InboxProvider, useInboxAdapter } from './inbox/provider';
+export { useInbox, useMailThread, useCompose } from './inbox/hooks';
+export { Inbox, MailReader } from './inbox/inbox';
+export type { InboxAdapter } from './inbox/adapter';
+export type { InboxItem, InboxState, MailAttachment, MailMessage, MailPerson } from './inbox/types';
+
 export type { MessagingAdapter } from './adapter';
 export type { ConversationsState } from './hooks/use-conversations';
 export type { MessagesState, UiMessage } from './hooks/use-messages';